This invention relates to apparatus for converting energy present in surface waves of large bodies of water into useful electrical energy.
Various wave energy converter (WEC) systems are known. For example, reference is made to U.S. patent application Ser. No. 09/379,421 filed Aug. 21, 1999, titled “Wave Energy Converter Utilizing Pressure Difference”, assigned to the assignee of the present application and the teachings of which are incorporated herein by reference.
Known WEC systems generally include a “float” (or “shell”) and a “spar” (or “shaft” or “column” or “piston”) which are designed to move relative to each other to convert the force of the waves into mechanical energy. In these systems, the float is generally depicted or referred to as the moving member and the spar as the non-moving or mechanically grounded member. But, the opposite may be the case. Alternatively, the spar and float may both move relative to each other.
In these prior art WEC systems, the float and spar are exposed to the water elements and forces. As shown in FIG. 1, a WEC generally includes a power-take-off device (PTO) coupled between the float (WEC shell) and the spar (shaft or column) to convert the mechanical power available from the WEC into electrical power. The PTO device may be any device capable of converting the relative motion between the float and spar into electrical energy. For example, the PTO device may be a linear-to-rotary translator (e.g. rack and pinion gear assembly, a ball screw assembly, a hydraulic cylinder and motor assembly, or a pneumatic cylinder and motor assembly) coupled to a rotary electric generator. The PTO device can also be a linear electric generator (LEG) that directly converts mechanical power to electric power using electromagnetic induction.
In some WEC systems the PTO device is placed in the water and is coupled to the float and spar. In other systems, a mechanical linkage (e.g. “pushrod”) connected to one of the float and spar is attached to a PTO device located inside the other of the float and spar, with the pushrod passing through an air-tight seal.
Numerous problems exist in the design of such systems for harnessing the energy contained in water surface waves. Some of these problems include:                The bearings between the float and spar are complex and expensive because of the need to operate in water and to be subjected to marine growth, contamination and corrosion.        The power take-off device and its bearings are complex and expensive because of the need to operate in water and be subjected to marine growth, contamination and corrosion.        The mechanical linkage connecting a float to an internally mounted PTO is subject to marine growth, corrosion and contamination.        Wave forces and viscous damping limit the extent to which the float and spar can move relative to each other, thereby decreasing the potential for energy collection. The efficiency of a “point absorber” type WEC is often limited by the viscous damping of the water.        The design of a mooring (anchoring) system for a WEC consisting of two or more objects that interact directly with the water and waves is often complex.Some of the problems noted above have been recognized and addressed in the prior art, as discussed, for example, in: (1) Temeev, A., Antufyev, B., and Temeev, S.; “Simulation of Oscillatory Drive for Float Wave Energy Converter”, in Fifth European Wave Energy Conference Proceedings, Hydraulics & Maritime Research Centre, Cork, Ireland, pp. 386-391, 2003; and (2) French, M. J. and Bracewell, R. H., “Heaving Point-Absorbers Reacting Against an Internal Mass”, in Hydrodynamics of Ocean Wave-Energy Utilisation, Lisbon, Portugal, Springer-Verlag, pp 247-55, 1985. As suggested in these references some of the problems, discussed above, may be overcome by constructing a WEC with a “float” that is acted upon by the waves, a “reaction” mass that is totally contained within the float, and a spring and power take-off device that couple the reaction mass to the float. In this type of system, the enclosed mass (m) is suspended from, or supported by, a spring that is connected to the float and whose force constant (k) is tuned to give the desired natural period (Tn) of the WEC.        
A problem with this approach (i.e., selecting the spring force characteristic to yield a desired natural period) is that the length of the spring is typically very large, and it is not practical to construct or house such a large spring within the float. The length of the spring in still water (x0) can be determined by solving the two following equations simultaneously.
                              m          ·          g                =                  k          ·          x                                    Equation        ⁢                                  ⁢        1                                                      k            m                          =                              f            n                    =                                    2              ⁢              π                                      T              n                                                          Equation        ⁢                                  ⁢        2            Equation 1 shows that the downward force of the reaction mass (m·g) is equal to the upward force of the spring (k·x) in static conditions. Equation 2 shows that the mass (m) and spring force constant (k) can be selected to give the mass-spring oscillator a natural oscillating frequency near that of the predominant waves; where Tn is equal to the period of the wave.
If the two equations are solved simultaneously, the still-water extension spring length (x0) would be:
                              x          0                =                                            (                                                T                  n                                                  2                  ⁢                                                                          ⁢                  π                                            )                        2                    ·          g                                    Equation        ⁢                                  ⁢        3            If the mass-spring system is tuned for a 4-second wave (T), the length of the spring (x0) would be approximately 4 meters. If the mass-spring system is tuned for an 8-second wave (T), the length of the spring (x0) would be approximately 16 meters. Fabricating and locating such a large spring within a float presents many problems.
The problem with the need for a very long spring, described above, is overcome in systems embodying the invention as described below.